The embodiments of the invention described herein relate generally to combustors, and to methods, tests, and devices for a multi-functional fuel converter. In particular, the embodiments relate to a multi-functional converter based on catalytic combustion and electrostatic injection technologies.
The Army has a need for small portable power systems to support a range of operations including but not limited to being carried by a soldier, or located in unmanned aerial vehicles (UAVs), remotely powered vehicles (RPVs), and unmanned ground vehicles (UGV). Typically, these power systems are battery operated, and as such, are limited with regard to power density, payload and performance duration. Thus, the use of combustion of energy dense liquid fuels such as, for example, ethanol, 1-butanol and Jet Propellant 8 (JP-8) in a compact manner is an attractive alternative to the battery option. Specifically, these dense liquid fuels function in a catalytic micro-combustor and combust at temperatures that can be used in energy conversion devices. However, although micro-combustors have the potential to outperform batteries, the current technology is limited with regard to system run time and reliability due to inefficient fuel-based energy conversion and poor fuel atomization at small flow rates (e.g., <10 mL/hour). As such, in order to improve system efficiency and reliability, the present invention, among other things, offers a catalytic fuel converter for small or micropower devices based on catalytic combustion and electrostatic injection technologies.
It is widely known by those having ordinary skill in the art that there are two parameters critical to the process of miniaturizing a combustor: time/length scales for complete evaporation/mixing and thermal management. Specifically, the time and length scales are vital because complete evaporation must occur in order for the gas-phase fuel and air to mix sufficiently for combustion on the catalyst. Further, maintaining autothermal combustion at the microscale requires catalytic combustion to reduce the temperature of stable combustion. The catalyst material is key because different catalysts are more catalytically active (i.e., have shorter reaction time scales) than others and as such, can produce higher catalytic conversions with lower surface areas. The smaller more effective catalysts require less space and can allow the reaction zone of the combustor to decrease in size, thereby affecting the overall size of the combustor.
The optimization of catalysts has been a major area of research. In particular, experiments have been conducted with catalysts composed of thermally stabilized, ion-exchanged zeolite, palladium on stabilized alumina, and catalysts doped with Cerium (Ce) and Nickel (Ni) to better prevent sulfur poisoning when using JP-8. It has been determined that catalysts can lower the activation barrier of a reaction, thereby allowing the reaction pathway to occur at lower temperatures than homogeneous reactions. This is extremely advantageous because it allows for combustion temperatures ranging between 650 K and 850 K, which are ideal for lead telluride (PbTe) thermoelectric devices. Conversely, homogeneous gas phase flame-combustion reactions of ethanol were carried out in the literature but typical reactor wall temperatures for these types of reactions can exceed 1200 K, which is far too high for any economically viable energy conversion device. At these high temperatures, construction materials for the microcombustor system may crack or melt and as such, the choice of materials becomes limited.
Oxygenates, such as ethanol and 1-butanol, are energy dense and well known to have characteristics that are desirable to miniaturize conventional combustors with regard to catalytic combustion. For instance, they are easily renewable from biomass, and possess simple straight-chain chemical structures. Further, ethanol and 1-butanol both become liquids at room temperature and boil at 351.4 K and 390.7 K, respectively. Additionally, the two alcohols are single components, not a combination of components having different boiling points and viscosities, which consequently enable simpler evaporation and diffusion models when attempting to predict their behavior. Further, they burn clean meaning complete combustion results in the production of CO2 and water. Moreover, it has been determined that both ethanol and 1-butanol are electrically conductive enough such that they can be injected using alternative fuel injection techniques, including but not limited to electrospraying techniques.
Electrospraying techniques are generally techniques that employ electrostatics to disperse a liquid (e.g., fuel) or a fine aerosol mist thereof. Further, the types of electrospraying techniques may include: electrospraying in cone-jet mode, multi-jet mode or charge injection methods using either a single nozzle electrospray (ES) or multiple nozzles (i.e., multiplexed electrospray (MES)) enable liquid atomization, evaporation, and mixing with the oxidizer at low temperatures. Low temperature fuel injection avoids pre-combustion in the fuel-air mixing zone (e.g., where the ethanol boiling point=351.4 K, flash point<300 K; and the 1-butanol boiling point=390.7 K, flash point=308 K) and reduces the requirement for pre-heating the air or fuel, thereby reducing overall power consumption and size of the combustor. Conversely, alternative techniques for fuel injection cannot produce the droplet size or distribution required for adequate evaporation and mixing within a time frame/distance required for compact burner applications. Furthermore, alternative fuel injection techniques typically require large flow velocities limiting them to high flow rate applications. To achieve the high flow velocities, conventional fuel injectors require substantial liquid pressurization that is not feasible for compact and low power burner applications.
Neither the combustion of ethanol nor 1-butanol poison the catalyst, and consequently operation using a single catalyst can persist for longer periods of time than, for instance, with fuels having high sulfur content. Further, the combustion of ethanol and 1-butanol are extremely exothermic and occur via Equations 1 and 2 respectively, shown below:CH3CH2OH+3O2→2CO2+3H2OΔH=−1407 kJ/mol  (1)CH3CH2CH2CH2OH+6O2→4CO2+5H2OΔH=−2713 kJ/mol  (2)
The equivalence ratio (φ) of a system, defined herein as the ratio of the fuel-to air/oxidizer ratio to the stoichiometric fuel-to air/oxidizer ratio, plays an important role in fuel conversion. Mathematically, the equivalence ratio is represented as:
                                                        ϕ              =                                                fuel                  ⁢                                      -                                    ⁢                  to                  ⁢                                      -                                    ⁢                  oxidizer                  ⁢                                                                          ⁢                  ratio                                                                      (                                          fuel                      ⁢                                              -                                            ⁢                      to                      ⁢                                              -                                            ⁢                      oxidizer                      ⁢                                                                                          ⁢                      ratio                                        )                                    st                                                                                                        =                                                                    m                    fuel                                    /                                      m                    ox                                                                                        (                                                                  m                        fuel                                            /                                              m                        ox                                                              )                                    st                                                                                                        =                                                                    n                    fuel                                    /                                      n                    ox                                                                                        (                                                                  n                        fuel                                            /                                              n                        ox                                                              )                                    st                                                                                        (        3        )            where m represents the mass flow rate, n represents the molar flow rate, and suffix st stands for stoichiometric conditions.
The (φ)-value can be controlled by adjusting the amounts of fuel and/or oxygen that are reacted. Having a φ-value of unity (1) signifies a stoichiometric feed of fuel and air, as shown in the above equations. With reactions having high φ-values (i.e., values ranging from 0.75 to 3), the reaction is considered “fuel rich” and incomplete combustion occurs because not enough oxygen exists to combust the fuel. However, reactions having low φ-values (i.e., values ranging from 0 to 0.75) indicate reactions having a “fuel lean” environment with plenty of oxygen to oxidize the fuel into its combustion products: carbon dioxide and water.
Various patents have been issued disclosing the use of a microcombustion-based thermoelectric generator. In particular, U.S. Pat. No. 6,951,456 to Adam L. Cohen et al. shows a design (e.g., a variation of a Swiss-roll type combustor) where the reactant channel and exhaust channels are coiled around each other in a spiral configuration so that heat loss is reduced. Some T-shaped or L-shaped fins are included as parts of the thermoelectric active wall. Additionally, some fins are extended in the reactant channel, while others are extended in the exhaust channel. These fins were designed to act as a diffusion barrier between the n-type and p-type Terillium (Te) materials and to increase the thermal gradient. However, in contrast to the embodiments described herein, the above mentioned patent is based on homogeneous combustion.
Additionally, published U.S. Patent Application 2007011146 discloses various catalytic converters to achieve complete combustion of liquid fuel using electrospray technologies with the cone-jet mode to produce fuel droplets in uniform size. It is to be noted however, that this previous patent investigated hydrocarbons and jet fuels as a source of fuel, while the present invention, among other things, utilizes oxygenates as fuel instead of hydrocarbons. This patent application also employed a thin mesh support layer for the catalyst limiting the potential production rates when compared to the foam support layer of the present invention. Further, the present invention demonstrates that a single reactor design is multi-functional and can operate in multiple modes including: reforming, bio-refinery, and combustion mode, as will be discussed in further detail below.